Feedback circuitry and methodology for switching circuit capable of generating both positive and negative output voltages

ABSTRACT

Circuitry arranged for sensing a variable polarity signal comprises an input node supplied with the variable polarity signal to produce an input signal. A first sensing circuit is responsive to the input signal having a first polarity for producing a first signal. A level shifting circuit is responsive to the input signal having a second polarity for shifting a level of the input signal to produce a shifted signal of the first polarity at a level determined by the input signal.

TECHNICAL FIELD

The subject matter of this disclosure relates to power supply circuits,and more particularly to circuitry and methodology for providing afeedback scheme for an integrated circuit (IC) having a switchingcircuit, such as a converter or switching regulator, that can be used togenerate both positive and negative output voltages. The feedback schemehas particular utility in the context of single-pin feedback of positiveand negative output voltages.

BACKGROUND

Some switching circuits, such as converters or switching regulators, maybe used to generate both positive and negative output voltages. Forexample, the LT®3471 dual switching regulator manufactured by LinearTechnology Corporation, the assignee of the present application,includes two channels that may be configured as two boost converters, aboost converter and an inverter, or two inverters. Accordingly, eachchannel may be configured into a non-inverting boost regulator toproduce positive output voltage or into an inverter to produce negativeoutput voltage.

To support both positive and negative output voltages, this IC requirespositive feedback pin FBP and negative feedback pin FBN for each channeland a reference pin Vref that may be shared by the channels. Asillustrated in FIG. 1, the positive and negative feedback pins FBP andFBN are respectively connected to non-inverting and inverting inputs ofan error amplifier EA that produces control voltage Vc at its output.Positive or negative output voltage Vout+ or Vout− is sensed using anexternal resistor divider composed of resistors R1 and R2. For thepositive output, FBP is connected to the reference voltage pin Vref, FBNis connected between R1 and R2. For the negative output, FBN is tied toground and FBP is connected between R1 and R2. If a negative output issensed, a change in phase is provided.

Another example is a feedback scheme used in the LT®1370 switchingregulator manufactured by Linear Technology Corporation, the assignee ofthe present application. This regulator can regulate positive ornegative output voltage. As illustrated in FIG. 2, it has positive andnegative feedback pins FB and NFB respectively provided for positive andnegative output voltage sensing. Also, it has negative and positiveerror amplifiers EA1 and EA2, respectively. NFB is connected to theinverting input of the negative error amplifier EA1. The non-invertinginput of EA1 is tied to ground. Via diode D, the output of EA1 iscoupled to FB connected to the inverting input of EA2. The non-invertinginput of EA2 is supplied with a 1.25V reference voltage.

Positive or negative output voltage Vout+ or Vout− is sensed using anexternal resistor divider composed of resistors R1 and R2. For positiveoutputs, NFB is shorted to FB which is regulated to 1.25V by theregulator loop. The output of EA1 is driven low, so the diode D is off.For negative outputs, EA1 drives FB to 1.25V so that NFB is regulated to0V by the regulator loop.

To reduce the number of external pins provided in a chip for controllinga switching circuit capable of producing positive and negative outputvoltages, it would be desirable to develop a feedback scheme using asingle pin for sensing both positive and negative output voltages.

SUMMARY OF THE DISCLOSURE

In accordance with one aspect of the disclosure, circuitry arranged forsensing a variable polarity signal comprises an input node supplied withthe variable polarity signal to produce an input signal. A first sensingcircuit is responsive to the input signal having a first polarity forproducing a first signal. A level shifting circuit is responsive to theinput signal having a second polarity for shifting a level of the inputsignal to produce a shifted signal of the first polarity at a leveldetermined by the input signal. The input node may be a pin extendingfrom a package housing the circuitry.

For example, the first sensing circuit may be responsive to a positivevoltage at the input, and the level shifting circuit may be configuredto shift a negative voltage level at the input to a positive voltagelevel corresponding to the negative voltage level. The level shiftingcircuit may include a PMOS or PNP circuit.

In accordance with an embodiment of the disclosure, a switch may becoupled to the input for supplying the input signal to the first sensingcircuit if the input signal has a first polarity, and for supplying theinput signal to the level shifting circuit if the input signal has asecond polarity. A threshold detector may compare the input signal witha pre-determined threshold value to control the switch.

A second sensing circuit may be responsive to a shifted signal at anoutput of the level shifting circuit for producing a second signal. Anoutput circuit may be responsive to the first and second signals toproduce an output signal.

The first sensing circuit may be a differential amplifier that comparesthe input signal with a first reference signal to produce the firstsignal that represents a difference between the input signal and thefirst reference signal. The second sensing circuit may be a differentialamplifier that compares the shifted signal with a second referencesignal to produce the second signal that represents a difference betweenthe shifted signal and the second reference signal.

The output circuit may include a transconductance amplifier to producean output current value. If a positive output is being sensed, thenegative sensing circuitry is disabled, so there is no second signalgenerated; similarly, if a negative output is being sensed, the positivesensing circuitry is disabled, so there is no first signal generated.

In accordance with another aspect of the disclosure, a system forcontrolling a switching circuit having an output for producing an outputvoltage of first and second polarities comprises a feedback circuit thatreceives the output voltage via a single input and produces a controlsignal responsive to the output voltage to control switching of theswitching circuit. The feedback circuit includes:

-   -   a first sensing circuit responsive to an input signal when the        output voltage has the first polarity for producing a first        signal representing the output voltage of the first polarity,    -   a voltage modification circuit responsive to the input signal        when the output voltage has the second polarity for modifying a        voltage level of the input signal from a first voltage level of        the second polarity to a second voltage level of the first        polarity representing the first voltage level, and    -   an output circuit responsive to output signals of the first        sensing circuit and the voltage modification circuit for        producing the control signal.

The feedback circuit may further comprise a switch for supplying theinput signal to the first sensing circuit when the output voltage hasthe first polarity, and for supplying the input signal to the voltagemodification circuit when the output voltage has the second polarity. Athreshold detector may compare the input signal with a threshold levelto control the switch.

The feedback circuit may further include a second sensing circuitcoupled to the voltage modification circuit for producing a secondsignal representing the output voltage of the second polarity.

The first sensing circuit may compare the input signal with a firstreference value to produce the first signal, and the second sensingcircuit may compare an output signal of the voltage modification circuitwith a second reference value to produce the second signal. The outputcircuit may produce the output signal representing a difference betweenthe first signal and the second signal.

In accordance with a method of the present disclosure, the followingsteps are carried out for sensing signals at an output of a circuitconfigurable for forming an output signal having a first or secondpolarity:

-   -   sensing the output signal to produce a sensed signal having a        polarity corresponding to the polarity of the output signal,    -   if the sensed signal has a first polarity, producing a        respective signal of the first polarity and    -   if the sensed signal has a second polarity, shifting a voltage        level of the sensed signal by a preset voltage value from a        first voltage level of the second polarity to a second voltage        level of the first polarity.

For example, the step of shifting may be carried out if the sensedsignal has a negative polarity.

Additional advantages and aspects of the disclosure will become readilyapparent to those skilled in the art from the following detaileddescription, wherein embodiments of the present disclosure are shown anddescribed, simply by way of illustration of the best mode contemplatedfor practicing the present disclosure. As will be described, thedisclosure is capable of other and different embodiments, and itsseveral details are susceptible of modification in various obviousrespects, all without departing from the spirit of the disclosure.Accordingly, the drawings and description are to be regarded asillustrative in nature, and not as limitative.

BRIEF DESCRIPTION OF THE DRAWINGS

The following detailed description of the embodiments of the presentdisclosure can best be understood when read in conjunction with thefollowing drawings, in which the features are not necessarily drawn toscale but rather are drawn as to best illustrate the pertinent features,wherein:

FIGS. 1 and 2 illustrate conventional feedback circuits in switchingregulators.

FIG. 3 illustrates general topology of a single-pin feedback circuit ofthe present disclosure.

FIG. 4 is a block-diagram illustrating an exemplary embodiment of thefeedback circuit of the present disclosure.

FIG. 5 is a circuit diagram illustrating exemplary implementation of thefeedback circuit in FIG. 4.

DETAILED DISCLOSURE OF THE EMBODIMENTS

The present disclosure will be made using the example of a switchingregulator. It will become apparent, however, that the concept of thedisclosure is applicable to sensing an output of any circuit capable ofproducing both positive and negative output signals.

FIG. 3 schematically illustrates a single-pin feedback circuit 100 ofthe present disclosure for controlling a switching regulator capable ofproducing positive and negative output voltages. The circuit 100 has afeedback pin FB that may protrude outside a package housing theregulator. The FB pin is supplied with sensed positive output voltageVout+ or sensed negative output voltage Vout−. For example, the positiveoutput voltage Vout+ may be produced when the switching regulator isconfigured into a boost regulator topology, and the negative voltageVout− may be generated when the switching regulator is configured intoan inverter topology.

The positive output voltage Vout+ may be sensed using a resistor dividercomposed of resistor R1 coupled to a source of voltage Vout+ andgrounded resistor R2. The negative output voltage Vout− may be sensedusing a resistor divider composed of grounded resistor R3 and resistorR4 coupled to a source of voltage Vout−. The feedback pin FB may beconnected to a common node between resistors of the resistor dividers.

Based on a variable polarity signal supplied to the feedback pin FB, thefeedback circuit 100 generates a control signal Vc that can be used tocontrol switching of the switching regulator to regulate the outputvoltage Vout+ or Vout−. For example, the control signal Vc may besupplied to an error amplifier that compares it with a preset referencesignal to generate an error signal for controlling apulse-width-modulation (PWM) switching control circuit. The controlsignal Vc may be generated as a voltage value Vc or as a current valuerepresenting the voltage Vc.

The feedback circuit 100 may be provided on a switching regulator chip.As disclosed in more detail below, this circuit requires only a singleexternal pin to support its operation. The sensing resistor dividers R1,R2 and R3, R4 may be arranged outside of the chip or may be provided onthe chip, together with the feedback circuit 100.

The feedback circuit 100 comprises a positive sensing channel 102 and anegative sensing channel 104. Their inputs are coupled to the inputfeedback pin FB for respectively sensing positive and negative polarityinput signals developed at the feedback pin FB. The outputs of thepositive and negative sensing channels 102 and 104 are connected to anoutput stage 106 that produces a control signal Vc.

FIG. 4 shows a block-diagram illustrating an exemplary embodiment of thesingle-pin feedback circuit 100 of the present disclosure. The feedbackpin FB may be connected to a switching device 202 that connects thepositive sensing channel to the FB pin if an input voltage detected atthe FB pin has a positive polarity, and connects the negative sensingchannel to the FB pin if the detected FB input voltage has a negativepolarity. The switching device 202 may be controlled by a thresholddetector 204 that compares the input voltage at the FB pin with athreshold voltage selected to detect when the input voltage becomes wellabove ground.

When a switching regulator that generates the sensed output voltage Voutstarts up, the input voltage at the FB pin is near the ground.Therefore, initially, the switching device 202 may connect the FB pin tothe negative sensing channel. When the FB input voltage exceeds thethreshold voltage, the threshold detector 204 detects that the voltagedeveloped at the FB pin is well above ground and produces a controlsignal to switch the FB pin to the positive sensing channel.

The positive sensing channel may include a positive sensing amplifier206 that compares the FB input voltage with a first reference voltageVref1. For example, the positive sensing amplifier 206 may be adifferential amplifier that produces a positive polarity signalrepresenting a difference between the FB input voltage and the firstreference voltage Vref1. The output signal of the positive sensingamplifier 206 may be supplied to an output stage 208 that produces acontrol signal Vc. For example, the output stage 208 may include atransconductance amplifier that produces current corresponding to thecontrol voltage value Vc. Alternatively, the output stage may include avoltage amplifier that produces control voltage Vc. The referencevoltage Vref1 may be selected so as to produce the control signal Vcthat accurately represents any positive input voltage detected at the FBpin.

If the input voltage at the FB pin does not exceed the thresholdvoltage, the threshold detector 204 controls the switching device 202 toconnect the FB pin to the negative sensing channel that includes a levelshifter 210 and a negative sensing amplifier 212. When the FB inputvoltage is below ground, the level shifter 210 shifts the input level toproduce a voltage above ground corresponding to the FB input voltage.The level shifter 210 may be configured to shift the input level by apreset value so as to transform a negative voltage at the FB pin intothe corresponding positive voltage. As a result, the level shifter 210produces a positive voltage that accurately represents any negativeinput voltage detected at the FB pin.

The output voltage produced by the level shifter 210 is supplied to thenegative sensing amplifier 212 that compares this voltage with a secondreference voltage Vref2. The negative sensing amplifier 212 may includea differential amplifier that produces a positive polarity signalrepresenting a difference between the output voltage of the levelshifter 210 and the second reference voltage Vref2. The output signal ofthe negative sensing amplifier 212 may be supplied to the output stage208 that produces a control signal Vc. For example, the output stage 208may include a transconductance amplifier. The reference voltage Vref2may be selected to produce the control signal Vc that accuratelyrepresents any positive voltage developed at the output of the levelshifter 210. For example, the first reference voltage Vref1 may be equalto the second reference voltage Vref2.

Hence, in response to an input signal of any polarity developed at theFB pin, the feedback circuit 100 produces a control signal Vc accuratelyrepresenting the FB input signal. Therefore, a single FB pin issufficient to support sensing of positive voltage Vout+ as well asnegative voltage Vout− that may be produced at the output of a switchingregulator.

FIG. 5 illustrates exemplary circuit implementation of the embodiment inFIG. 4. In this example, the feedback circuit 100 includes CMOStransistors M1-M3, bipolar transistors Q1-Q11 and resistors R1-R15.However, one skilled in the art would realize that the circuit 100 mayalso be implemented using only a CMOS or BiCMOS technology, or usingonly a bipolar technology.

The switching device 202 and the threshold detector 204 may be providedusing a circuit including an NMOS transistor M3 and a pair of NPNtransistors Q3 and Q4. The gate of M3 is connected to the feedback pinFB. A 20K resistor R5 may be coupled to the transistor Q4.

The positive sensing amplifier 206 may include a pair of NPN transistorsQ1 and Q2 arranged in a differential amplifier configuration. The baseof Q1 may be connected to the FB pin. The base of Q2 may be suppliedwith a 1.25V Bandgap reference voltage.

The level shifter 210 may comprise PMOS transistors M1 and M2 connectedtogether with resistors R3 and R4 between the ground and a 1.6Vreference voltage node. The 1.6 V reference voltage may be provided byboosting a Bandgap reference voltage. The gate of M2 may be connected tothe gate of M3. The gate of M1 may be grounded. 20K resistors may beused as resistors R3 and R4.

The negative sensing amplifier 212 may include a pair of PNP transistorsQ5 and Q6 arranged in a differential amplifier configuration. The baseof Q5 is connected to the output of the level shifter M1, M2. The baseof Q6 is supplied with the 1.6V reference voltage via a resistor dividerR14, R15, which may be composed of 40K resistors.

The NPN differential pair Q1, Q2 and the PNP differential pair Q5, Q6are connected to the output stage 208 that may be implemented using atransconductance amplifier including PNP transistors Q9-Q11 and NPNtransistors Q7, Q8. The control signal Vc is produced at a nodeconnected to collectors of Q8 and Q11. Resistors R9-R11 may be connectedfor supplying a 2.5V power supply voltage to emitters of transistorsQ9-Q11, respectively. For example, R9 may be a 2K resistor, R10 and R11may be 5K resistors. Bases of Q9-Q11 may be grounded via an 85K resistorR12. The emitters of transistors Q7 and Q8 may be grounded via 5Kresistors R7 and R8, respectively.

For positive output voltages Vout+, the input voltage at the FB pin isinitially at a level close to ground. As a result, the PNP differentialpair Q5, Q6 is connected to the FB pin to drive the Vc pin. When, the FBinput voltage becomes high enough to exceed the preset thresholdvoltage, the PNP pair Q5, Q6 turns off, and the NPN differential pairQ1, Q2 takes over and regulates the control signal Vc like a traditionalerror amplifier.

For negative output voltages Vout−, the NMOS transistor M3 is turned offand the PNP differential pair Q5, Q6 controls the Vc pin. The PMOStransistors M1, M2 and resistors R3, R4, provides level shifting byduplicating a 1.6V voltage (V_(GS,M1)+V_(R3)) across a circuit composedof the gate-source junction of M2 and resistor R4, where V_(GS,M1) isthe voltage between the gate and source of M1 and V_(R3) is the voltageacross R3. As a result, a −0.8V regulation point for the FB pin iscreated at the gate of M2. The transistors M1 and M2 may have identicaldrain current I_(D), gate-source voltage V_(GS), drain-source voltageV_(DS) and drain-gate voltage V_(DG) to accurately match each other atthe regulation point.

It is noted that the resistor R5 plays an important role in how smoothlytransition between the NPN and PNP pairs occurs. The value of R5 may beselected high enough to avoid zero output current at the Vc node duringthe transition, and low enough to ensure that the Q5, Q6 pair arecompletely cut off for positive outputs Vout+.

The level of the regulation point for negative outputs Vout− may beselected in accordance with a reference voltage supplied to the negativesense amplifier. For example, this level may be set at a half of thereference voltage, which may be made higher than the maximum thresholdvoltage V_(TH,MAX) of a PMOS transistor in the level shifter so thatenough voltage can be generated across R3 to ensure good matching oftransistors M1 and M2, and resistors R3 and R4. The level of theregulation point may also be made lower than the minimum thresholdvoltage V_(TH,MIN) of a PMOS transistor in the level shifter to keep M1and M2 in saturation.

The foregoing description illustrates and describes aspects of thepresent invention. Additionally, the disclosure shows and describes onlypreferred embodiments, but as aforementioned, it is to be understoodthat the invention is capable of use in various other combinations,modifications, and environments and is capable of changes ormodifications within the scope of the inventive concept as expressedherein, commensurate with the above teachings, and/or the skill orknowledge of the relevant art.

The embodiments described hereinabove are further intended to explainbest modes known of practicing the invention and to enable othersskilled in the art to utilize the invention in such, or other,embodiments and with the various modifications required by theparticular applications or uses of the invention.

1. Signal sensing circuitry arranged on a chip, for producing an outputsignal to control a circuit, the signal sensing circuitry comprising: aninput node configured for receiving an input signal supplied from anoutput of the controlled circuit, the input signal having a variablepolarity and being provided externally with respect to the chip, a firstsensing circuit responsive to the input signal having a first polarityfor producing a first signal, and a level shifting circuit responsive tothe input signal having a second polarity for shifting a level of theinput signal to produce a shifted signal of the first polarity at alevel determined by the input signal.
 2. The circuitry of claim 1,wherein the first sensing circuit is responsive to a positive voltage atthe input node, and the level shifting circuit is configured to shift anegative voltage level at the input node to a positive voltage levelcorresponding to the negative voltage level.
 3. The circuitry of claim2, wherein the level shifting circuit includes a PMOS circuit.
 4. Thecircuitry of claim 2, wherein the level shifting circuit includes a PNPcircuit.
 5. The circuitry of claim 1, further comprising a switchcoupled to the input node for supplying the input signal to the firstsensing circuit if the input signal has the first polarity, and forsupplying the input signal to the level shifting circuit if the inputsignal has the second polarity.
 6. The circuitry of claim 5, furthercomprising a threshold detector for comparing the input signal with apre-determined threshold value to control the switch.
 7. The circuitryof claim 1, further comprising a second sensing circuit responsive tothe shifted signal for producing a second signal.
 8. The circuitry ofclaim 7, further comprising an output circuit responsive to the firstand second signals to produce the output signal.
 9. The circuitry ofclaim 8, wherein the first sensing circuit is configured to compare theinput signal with a first reference signal to produce the first signalproportional to a difference between the input signal and the firstreference signal.
 10. The circuitry of claim 9, wherein the secondsensing circuit is configured to compare the shifted signal with asecond reference signal to produce the second signal proportional to adifference between the shifted signal and the second reference signal.11. The circuitry of claim 10, wherein the output circuit includes atransconductance amplifier to produce an output current value.
 12. Asystem for controlling a switching circuit having an output forproducing an output voltage of first and second polarities, comprising:a feedback circuit arranged on a chip and configured for receiving theoutput voltage provided externally with respect to the chip, via asingle input node and responsive to the output voltage for producing acontrol signal to control switching of the switching circuit, thefeedback circuit including: a first sensing circuit responsive to theoutput voltage sensed at the input node when the output voltage has thefirst polarity, for producing a first signal representing the outputvoltage of the first polarity, a voltage modification circuit responsiveto the output voltage sensed at the input node when the output voltagehas the second polarity, for modifying a voltage level of the outputvoltage from a first voltage level of the second polarity to a secondvoltage level of the first polarity representing the first voltagelevel, and an output circuit responsive to output signals of the firstsensing circuit and the voltage modification circuit for producing thecontrol signal.
 13. The system of claim 12, wherein the feedback circuitfurther comprises a switch for supplying the output voltage to the firstsensing circuit when the output voltage has the first polarity, and forsupplying the output voltage to the voltage modification circuit whenthe output voltage has the second polarity.
 14. The system of claim 13,wherein the feedback circuit further comprises a threshold detector forcomparing the output voltage with a threshold level to control theswitch.
 15. The system of claim 12, wherein the feedback circuit furtherincludes a second sensing circuit coupled to the voltage modificationcircuit for producing a second signal representing the output voltage ofthe second polarity.
 16. The system of claim 15, wherein the firstsensing circuit is configured for comparing the output voltage with afirst reference value to produce the first signal, and the secondsensing circuit is configured for comparing an output signal of thevoltage modification circuit with a second reference value to producethe second signal.
 17. A method of sensing signals using sensingcircuitry arranged on a chip and having a node for receiving an outputsignal of a circuit configurable for forming the output signal having afirst or second polarity, the output signal being provided externallywith respect to the chip, the method comprising the steps of: sensingthe output signal at the node, if the sensed output signal has a firstpolarity, producing a signal of the first polarity and if the sensedoutput signal has a second polarity, shifting a voltage level of thesensed output signal by a preset voltage value from a first voltagelevel of the second polarity to a second voltage level of the firstpolarity, and in response to the output signal sensed at the node,controlling the circuit for forming the output signal.
 18. The method ofclaim 17, wherein the step of shifting is carried out if the sensedoutput signal has a negative polarity.
 19. A switching regulatorconfigured on a chip for producing an output voltage of first and secondpolarities, comprising: feedback circuitry coupled to a feedback nodeconfigured for receiving the output voltage of both the first polarityand the second polarity provided externally with respect to the chip,the feedback circuitry including: a first sensing circuit responsive tothe output voltage sensed at the feedback node when the output voltagehas the first polarity for producing a first signal representing theoutput voltage of the first polarity, a voltage modification circuitresponsive to the output voltage sensed at the feedback node when theoutput voltage has the second polarity for modifying a voltage level ofthe output voltage from a first voltage level of the second polarity toa second voltage level of the first polarity representing the firstvoltage level, and an output circuit responsive to output signals of thefirst sensing circuit and the voltage modification circuit for producinga control signal to control switching of the switching regulator.